Measuring devices

ABSTRACT

A combined toroid/shunt device for detecting residual current in an electrical installation comprising a plurality of conductors such as a live conductor ( 2 ) and a neutral conductor ( 3 ) is described. The device comprises a toroid means ( 1, 4 ) for detecting an AC residual current within a first range and a plurality of resistive shunts ( 6   a ) to ( 6   d ) for connection in respective ones of the plurality of conductors. A current detection means ( 5, 7, 13 ) responsive to current flowing in each of said shunts for detecting a DC residual current and/or an AC residual current within a second range is described. The first range of AC residual current is an AC residual current resulting from earth leakage or cross-leakage between conductors up to a saturation level at which the toroid or electronic means associated means therewith becomes saturated. The second range includes an AC residual current of said saturation level.

This invention relates to measuring devices for electricalinstallations, and in particular to measuring devices including acurrent/voltage detection module for analysing current and voltage tofacilitate, inter alia, a residual current detection and powerconsumption.

A method of detecting for residual current may involve using a currenttransformer having primary windings through which, in the case of asingle phase device, load current flows in opposite directions so thatif the return current is different from the outwardly flowing currentbecause of current leakage, an output current signal is induced in asecondary winding of the transformer. In the case of a multi-phasedevice, primary windings of the transformer are connected in all of thephase lines and the neutral line. In normal situations, when there is nocurrent leakage, the net current induced in the secondary winding iszero and therefore no output is detected. These devices are subject tonuisance tripping arising from surges in the supply, switches inappliances and the like. Further problems arise because the transformeris designed to be sensitive to very small current imbalances caused bycurrent leakage. With relatively large current leakage faults, themagnetic flux may cause the transformer core to become saturated and sofail to induce a current in the secondary winding. Alternatively, alarge induced current may cause saturation of an amplifier in theelectronic circuit which is used to process the induced current signal.

Also, toroidal transformer devices may be insensitive to dc currentleakage faults, such that the fault goes undetected and no trip occurs.Many electrical systems incorporate switching power supplies, forexample ac to dc converters and inverters in motor speed control andstart-up systems. In such systems the ac supply phases are switchedelectronically (for example with high voltage FETs) to provide rectifiedwaveform signals. In such cases a current leakage fault may not inducesufficient current in the secondary winding to detect the fault.

One method of determining power consumption is to measure the voltageacross the power supply wires and the current flowing through them andthen multiply the current by the voltage measurement to determine apower measurement. One approach is to use a shunt of known valueconnected in series with one of the wires and to measure the voltageacross and current flowing through it. Power meters include a counter orclock for measuring the number of watt-hours consumed. The counter orclock is periodically read manually in order that the consumer can bebilled for the quantity of electricity used.

Shunt resistors could also be used to detect an imbalance in the currentcaused by a current leakage. However, to be useful on their own as aresidual current detection safety device for tripping a circuit breaker,the shunt resistors would have to be extremely accurate. The currentflowing through the shunts would need to be detected to an accuracy inthe order of 1 to 10 mA in 100 A (10⁻⁵ to 10⁻⁴). This means thatsophisticated and complex measurement circuitry would be needed toprovide the required resolution as well as precise and stable shuntresistors having linear resistance characteristics.

Conventionally, residual current devices and power consumption metersare separate discrete devices. The power consumption meter is usuallylocated at the point of entry of the electricity supply into thepremises and the residual current device is located within the consumerunit or fuse box from which the circuits to the premises aredistributed.

It is an aim of the present invention to devise a combined residualcurrent and power measuring device which overcomes or at leastalleviates these problems. It is a further aim of the invention todevise a combined residual current and power measuring device which isless bulky than conventional devices and operates to an improved degreeof sophistication such as to facilitate remote monitoring of theelectrical installation.

According to a first aspect of the present invention, there is provideda measuring device for an electrical installation comprising a pluralityof conductors, the device comprising toroid means for detecting aresidual current and power consumption means comprising a shunt detectormeans for generating a current signal indicative of current detected inat least one of said conductors and a resistor detector means forgenerating a voltage signal indicative of voltage across at least onepair of said conductors.

In a preferred embodiment the electrical installation comprises an acsupply via a neutral conductor and a live conductor, said toroid meanscomprising a toroidal transformer detector for generating a residualcurrent signal in response to a detected residual current in saidelectrical installation, and said resistor detector means is providedfor connection between said neutral and live conductors to generate saidvoltage signal by measuring the voltage drop across the resistordetector means or a potentially divided portion thereof, the devicefurther comprising a processor means for generating a trip signalindicative of the presence of a residual current fault in dependence onsaid residual current signal, thereby to facilitate operation of acircuit breaker to break said ac supply in response to said trip signal,and an output signal derived from said current and voltage signals tofacilitate determination of power consumption.

In another preferred embodiment, the ac supply may comprise a pluralityof phases, each phase comprising a supply via a phase live conductor andsaid neutral conductor, wherein the shunt detector means comprises arespective shunt connected in series in each of said phase liveconductors for providing a respective current signal indicative of thecurrent flowing in the respective phase live conductor, and saidresistor detector means comprises a respective resistor means connectedbetween each of said phase live conductors and said neutral conductor,said processor being operative for generating signals representative ofthe voltage between each phase live conductor and said neutral conductorby measuring the voltage across each of said resistor means orpotentially divided portions thereof.

According to a second aspect of the present invention, there is provideda measuring device for an electrical installation having an ac supplyvia a neutral conductor and at least one live conductor, the devicecomprising a toroidal transformer detector for generating a residualcurrent signal in response to a detected residual current in saidelectrical installation, a shunt resistor detector for generatingrespective current signals indicative of current detected in saidneutral conductor and each of said at least one live conductors, aprocessor means for generating a trip signal indicative of the presenceof a residual current fault in dependence on said residual currentsignal and/or said respective current signals detected, and a circuitbreaker operative to break said ac supply in response to said tripsignal.

In a preferred embodiment, for each of said at least one liveconductors, the device may further comprise a resistor detector meansfor connection between said neutral and live conductors and operativefor providing a signal representative of the voltage between saidneutral and live conductors by measurement of the voltage drop acrosssaid resistor detector means or a potentially divided portion thereof.

In an embodiment of the invention, the processor means comprises a firstanalog to digital converter coupled to a secondary winding of saidtransformer for generating said residual current signal as a digitalsignal representative of the voltage sensed across the winding and/orthe current in the winding.

The processor means may further comprise a second analog to digitalconverter coupled to the shunt resistor detector for generating digitalsignals representative of the current flowing through said neutralconductor and each of said at least one live conductors.

The first and/or second analog to digital converter may also be coupledto said resistor detector means for generating digital signalsrepresentative of said voltage between said neutral and live conductors.

Alternatively, the first and/or second analog to digital converters mayinclude a multiplexer for selectively coupling two or more of saiddetector means and generating corresponding digital signalsrepresentative of the voltage or current detected.

Each analog to digital converter may include a delta-sigma modulator.

The processor means may include a microprocessor for receiving thedigital signals from the first and second analog to digital convertersfor determining the power consumed by the electrical installation fromthe digital signals. The microprocessor may be further operative forgenerating a current imbalance signal indicative of the residual currentduring real time from the residual current signal. The microprocessormay be further operative for generating said current imbalance signalfrom a comparison of said current signals indicative of the current insaid neutral and live conductors. The microprocessor may be furtheroperative for generating said trip signal on the basis of a comparisonof said current imbalance signal with a predetermined thresholdcriterion.

The microprocessor may be yet further operative for analysing theresidual current, current and voltage in order to detect one or moreother conditions, including, overcurrent, arc fault, standing currentleakage, and “True power” measurement from the phase angle(Power=Voltage * Current * Cosine (Phi)). The power consumption,together with the other operating conditions or events may be logged forfuture reference. This information is useful for diagnostic purposes.

A temperature sensor may be provided to allow for compensation fortemperature fluctuations in the shunts. The microprocessor may becalibrated to generate current and voltage signals taking into accountthe temperature of the shunts relative to a reference point.

The microprocessor may be arranged to adjust the threshold of theresidual current necessary to generate a trip signal if it learns thatthe residual current is caused by a standing leakage at theinstallation. The residual current detector function will thereforecontinue to operate as a safety device while minimising the possibilityof nuisance tripping. For example, if there were a standing leakage of10 mA when the monitoring device is installed, the device would tripwhen the predetermined threshold is reached above the 10 mA level ratherthan zero.

A communication device may be provided for transmitting this informationto a remote monitoring station.

According to another aspect of the present invention there is provided acombined toroid/shunt device for detecting a residual current in anelectrical installation comprising a plurality of conductors, the devicecomprising: a toroid means for detecting an ac residual current in afirst range; a plurality of resistive shunts for connection inrespective ones of said plurality of conductors; and current detectionmeans responsive to current flowing in each of said shunts for detectinga de residual current and/or an ac residual current in a second range.

In a preferred embodiment the first range of ac residual current is anac residual current resulting from earth leakage or cross-leakagebetween the conductors up to a saturation level at which the toroid orelectronic means associated therewith become saturated, and the secondrange includes ac residual currents above said saturation level.

The conductors may comprise a live conductor and a neutral conductor.Alternatively the conductors may comprise a neutral conductor and aplurality of live conductors in an installation having a multi-phasesupply.

It is evident that, in embodiments of the invention, the device isprovided with two means for detecting a residual current. The toroidaltransformer facilitates detection of a current imbalance between theneutral and live conductors, indicative of a residual current, from thevoltage induced in the secondary winding. The toroidal transformer candetect a residual current of a very low level (10 mA or less). The shuntdetector facilitates detection of a current imbalance from a comparisonof the current detected in the neutral and live conductors. The shuntdetector may thereby facilitate detection of a residual current evenwhen the residual current arises from a dc fault, not detectable by thetransformer. The shunt detector may also facilitate detection ofresidual current in circumstances where the magnetic flux causes thetransformer to saturate.

It is evident that embodiments of the invention provide for an integralor combined power and residual current device. One advantage arising outof this is that the power meter, which is usually owned by theelectricity supplier, may be owned by the user. Parameters relating toelectricity usage and fault conditions are transmitted to a stationmonitored by the supplier for billing, diagnostic, consumption orservice purposes. It is also apparent that a reduction in bulk relativeto conventional power measuring and separate residual current devices ispossible. This allows devices embodying the invention to be embedded inappliances to provide for trip avoidance, greater discrimination,failure indication and downstream monitoring. Sub-circuit metering maybe effected particularly if the devices are networked, this enablingbetter discrimination and isolation of faults, tripping the sub-circuitfirst, before the main circuit trips. Further uses are envisaged such asproviding warning data prior to tripping and use in building managementsystems.

The invention will now be further described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a combined power meter and residual currentdevice embodying the present invention applied to a single phaseelectricity supply;

FIG. 2 is a block diagram of a combined power meter and residual currentdevice embodying the present invention applied to a three phaseelectricity supply; and

FIG. 3 is a flow chart indicating an operating sequence for residualcurrent detection in a device embodying the present invention.

One structural configuration of a monitoring device embodying thepresent invention is shown in FIGS. 1 and 2. In a first aspect, FIG. 1shows a single phase device in which a toroidal transformer core 1 iscoupled to live and neutral conductors 2 and 3 respectively of an acmains supply. A secondary coil 4 is wrapped around the core 1 andcoupled to a first analog to digital converter 5 for generating adigital output O1 representative of the imbalance current sensed by thetoroidal transformer 1.

A shunt detector means comprises a resistive shunt 6 a provided inseries with the mains neutral conductor 3. This may be of a resistivematerial such as manganin having a nominal resistance of 0.2 mΣ to atolerance of less than 5%. Respective ends of the shunt 6 a areconnected to an analog to digital converter 7 which produces a digitisedoutput O2 representative of the voltage drop across the shunt 6 a. Thevoltage drop across shunt 6 a provides a measure of the current flowingin the neutral conductor 3.

A resistor detector means comprises a potential divider having resistors8 a, 8 b, 9 connected between the mains live and neutral conductors 2, 3respectively. The voltage between these conductors can be determined bymeasuring the voltage drop across the resistor 8 a by connectingrespective ends of the resistor 8 a to the second analog to digitalconverter 7. The digitised output O2 contains the information on thevoltage across the resistor 8 a.

A power supply unit 10 is provided for drawing power from the live andneutral mains conductors 2, 3 and for supplying controlled voltages tothe analog to digital converters 5, 7 via isolation barriers 11, 12 andprocessor 13. A multi-plexer may be provided in each converter forproviding to the processor, through the respective isolation barrier,signals representing both the current in the associated shunt and thevoltage at one end of it. The processor 13 uses these signals to monitorthe current in each shunt as well as the imbalance current sensed by thecoil 4 of the toroidal transformer 1.

In the event of a current imbalance exceeding a predetermined threshold,the processor generates a trip signal O3 which drives a solenoidactuator 14 for breaking or tripping respective conductors 2, 3 via acircuit breaker indicated schematically by switches 15 and 16 in FIG. 1.

The processor 13 is also provided with a Universal Asynchronous ReceiverTransmitter (UART) 25 as a means for generating and transmitting anoutput signal in the form serial data communications O4. The processor13 provides output signals based on the current and voltage signalsdetected. This may include a power consumption signal based on powercalculated from the detected current and voltage signals together with atime derived from a signal from a clock generator 26 within theprocessor 13.

Each one of the analog to digital converters comprises an analog todigital converter in the form of a delta-sigma modulator 17 whichprovides a high frequency one bit digital data stream. A temperaturesensor 18 is provided so that the digitised output signals O1 and O2 aremodified to compensate for temperature fluctuations. The modificationmay be effected by means of a calibration technique involving the use ofa look up table (not shown). The temperature compensation may take theform of a polynomial fitted to calibration test results, the polynomialcoefficients being stored in the look-up table.

In a second aspect, also shown in FIG. 1, the shunt detector meanscomprises a further shunt resistor 6 b provided in series with the mainslive conductor 2. Respective ends of the further shunt 6 b are connectedto analog to digital converter 5 such that digitised output O2 containsa digital signal representative of the voltage drop across the furthershunt 6 b. The voltage drop across further shunt 6 b provides a measureof the current flowing in the live conductor 2. The processor 13performs a comparison of the detected currents in the live and neutralconductors 2, 3 to detect a residual current. A residual current notdetected by the toroidal transformer 1, for example a dc residualcurrent or a saturating residual current, will be detected by the shuntdetector, from which the processor 13 generates trip signal O3.

The resistors 8 a, 8 b and 9 that comprise the resistor detector meansprovide an additional voltage signal by connecting respective ends ofthe resistor 8 b to the first analog to digital converter 5. Thedigitised output O1 contains the information on the voltage across theresistor 8 b. The resistors 8 a, 8 b, across which connections are madeto provide the voltage signals, are precision resistors of relativelylow ohmic value, while the intermediate resistor 9 has a relatively highohmic value. The ratio of the voltages measured across precisionresistors 8 a 8 b should remain constant. By monitoring this ratio, anindependent reference is provided, so that if the ratio changes overtime due to drift in the analog to digital converter 5 or itsreferences, an adjustment can be made by software within processor 13 tocorrect the value of the measured voltage.

FIG. 2 shows a three phase arrangement in which features similar tothose of FIG. 1 have a same reference numeral. In this case, the acsupply has two additional live conductors 21, 22 for the second andthird supply phases respectively. Two additional analog to digitalconverters 14, 15 are provided for the additional two phases. Thesegenerate outputs representative of the sensed current and voltage forthe second and third phases respectively and supply these to theprocessor 13 via isolation barriers 19 and 20. In the arrangement ofFIG. 2, the shunt detector means comprises resistive shunt 6 a in themains neutral conductor 3, and resistive shunts 6 b, 6 c, 6 d in each ofthe mains live conductors 2, 21, 22 respectively. It is noted that thevoltage sensing connections to analog to digital converters 23 and 24are made via resistor chains connected between each phase line and theneutral in a similar manner to the resistors 8 a, 8 b and 9 of FIG. 1.

In both FIG. 1 and FIG. 2, the processor 13 is programmed to carry outthe necessary calculations to determine the existence of an imbalance.It is also programmed to determine the current and voltage in respect ofeach phase to a high degree of precision for a subsequent powermeasurement. These measurements may be analysed in order to detect oneor more other operating conditions including arc fault, standing currentleakage, True power measurement. This information is useful fordiagnostic purposes.

Referring to FIG. 3, the operating procedure of a monitoring deviceincludes certain functions that are performed by hardware components andothers that are performed by a software program in the processor whichcomprises a micro-controller unit (MCU). At step 101 the power supply isactivated. Various checks are performed by the hardware to ensure thatthe supply is not switched on in the presence of a large residualcurrent. A power supply unit monitor 102 checks that the power supply isstable, if not the device waits until the supply is stable beforeproceeding to step 104 where a check is made that the clock in the MCUis stable. Once stability has been confirmed buy these checks the MCU isreset at step 107. If the checks at steps 103 and 104 do not confirmstability such that the MCU is reset within a predetermined time, thenat step 105 a watchdog timeout 106 provides a signal to operate asolenoid at step 108 that isolates the power supply.

Once the MCU has been reset, software in the MCU performs a calibrationof the analog to digital converters at step 109. The calibration usespredetermined criteria so that the analog current and voltage signalsmeasured are converted into digital signals representing thecurrents/voltages with the required precision.

Operating standards to which residual current devices are required tocomply are usually defined in terms of RMS current values. Therefore theprocessor calculates the RMS values of the voltages and currentsdetected. To evaluate an RMS value accurately, the calculation must beperformed over a full signal cycle or an integer multiple of signalcycles. This may be done by using a known supply frequency, or bymeasuring the supply frequency, for example by performing a Fourieranalysis on a sample of measured values. Alternatively the RMScalculation may be peformed over a specific time interval which containsan integer multiple of cycles for all rated operating frequencies. Ineither case, at step 110 the MCU must initialise the RMS values andtiming means.

A new RMS value can be calculated for each input waveform cycle or aftereach instantaneous measurement. The former method obtains a new RMSvalue every cycle and the latter after every measurement sample. Thelatter method is preferred as it gives a faster response time to anysuddenly appearing residual current or overcurrent. In either case ittakes at least 20 ms (rated frequency=50 Hz) to obtain the first RMSvalue. This is a significant portion of the time allowed for tripping onpower up in the presence of a residual current, so it may beadvantageous to have a specific routine at power up which looks atinstantaneous values rather than RMS.

Many mains measurement systems sample at frequencies to include the31^(st) harmonic. Allowing for operation up to 60 Hz and a frequencytolerance of ±5% gives a minimum sampling frequency of 3906 Hz, so asampling frequency of 4 KHz is often used, and this would seem a goodbasis for initial design. However, to ensure an integer number ofsamples at both rated mains frequencies of 50 Hz and 60 Hz a samplingfrequency of 3-9 Khz or 4-2 KHz is preferred.

The MCU, at step 111, evaluates the currents and voltages from thedetected signals, at step 112 calculates the residual current I, and atstep 113 calculates the RMS values.

At steps 114 to 117 the MCU performs various calculations andcomparisons for determining whether an unsafe condition in the form of aresidual current or other predefined condition exists. In the presenceof an unsafe condition a trip signal is generated so as to operate thesolenoid at step 108 to isolate the supply. The calculations andcomparisons performed will depend on the type of residual current deviceemployed. For example the comparisons shown at steps 115 and 117 wouldonly be suitable for use with a residual current operatedcircuit-breaker having over-current protection (RCBO).

Examples of the parameters calculated by the MCU include the following:

Primary Measurement Quantities

Instantaneous line and neutral current at 4.2 KHz, i_(1s), i_(2s),i_(3s) and i_(ns)

Instantaneous line voltage at 4.2 KHz, v₁, v₂, & v₃

Instantaneous Temperature at 4.2 KHz T₁, T₂ T₃ & T_(n)

Instantaneous Residual Current at 4.2 KHz (Toroid) i_(Δ) _(t) ,

Secondary Measurement Quantities

The secondary measurement quantities calculated by the MCU are definedas follows:

Instantaneous current, i_(Δ) _(s) , where this is the sum of thecurrents in all four shunts (n denoting neutral shunt)i _(Δ) _(s) =i _(1s) +i _(2s) +i _(3s) +i _(ns)

Instantaneous line and neutral current squared, i² _(1s), i² _(2s), i²_(3s) and i² _(ns)

Instantaneous line voltage squared, v² ₁, v² ₂, & v² ₃

Instantaneous power, p₁, p₂, p₃:

-   -   p₁=i_(1s).v₁, p₂=i_(2s).v₂ and p3=i_(3s).v₃

RMS residual current I_(Δ) _(s) (from shunts):$I_{\Delta\quad s} = \sqrt{\frac{1}{N} \cdot ( {\sum\limits_{j = 1}^{N}( {i_{\Delta\quad s}(j)} )^{2}} )}$where j is the index of the sample point within a whole cycle.

RMS residual current I_(Δ) _(t) (from Toroid):$I_{\Delta\quad t} = \sqrt{\frac{1}{N} \cdot ( {\sum\limits_{j = 1}^{N}( {i_{\Delta\quad t}(j)} )^{2}} )}$where j is the index of the sample point within a whole cycle.

Total line and neutral current (from shunts) I_(1s), I_(2s), I_(3s) andI_(ns):$I_{1s} = \sqrt{\frac{1}{N} \cdot ( {\sum\limits_{j = 1}^{N}( {i_{1s}(j)} )^{2}} )}$and similarly for I_(2s), I_(3s) and I_(ns).

RMS line voltage V₁, V₂ and V₃:$V_{1} = \sqrt{\frac{1}{N} \cdot ( {\sum\limits_{j = 1}^{N}( {v_{1}(j)} )^{2}} )}$

RMS real power$P_{1} = \sqrt{\frac{1}{N} \cdot ( {\sum\limits_{j = 1}^{N}( {v_{1} \cdot i_{1s}} )^{2}} )}$P₁, P₂  and  P₃:

RMS total power W₁, W₂ and W₃ where W₁=I_(1s).V₁

Phase angle for each line φ1, φ2 and φ3:$\phi_{1} = {a\quad{\cos( \frac{P_{1}}{W_{1}} )}}$In the definition of all secondary RMS quantities, N is the number of4.2 KHz output words in a line voltage cycle. N=84 for 50 Hz linefrequency and N=70 for 60 Hz.

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 15. (canceled)16. A measuring device for an electrical installation having an ACsupply via a neutral conductor and at least one live conductor, thedevice comprising a toroidal transformer detector for generating aresidual current signal in response to a detected residual current insaid electrical installation, a shunt resistor detector for generatingrespective current signals indicative of current detected in saidneutral conductor and each of said at least one live conductors, aprocessor means for generating a trip signal indicative of the presenceof a residual current fault in dependence on said residual currentsignal and/or said respective current signals detected, and a circuitbreaker operative to break said AC supply in response to said tripsignal.
 17. A measuring device according to claim 16, further comprisingresistor detector means for connection between said neutral and liveconductors and operative for providing a signal representative of thevoltage between said neutral and live conductors by measurement of thevoltage drop across said resistor detector means or a potentiallydivided portion thereof.
 18. A measuring device according to claim 16,wherein the processor means comprise a first analog to digital convertercoupled to a secondary winding of said transformer for generating saidresidual current signal as a digital signal representative of thevoltage sensed across the winding and/or the current in the winding. 19.A measuring device according to claim 18, wherein the first analog todigital converter includes a multiplexer for selectively coupling two ormore of said detector means and generating corresponding digital signalsrepresentative of the voltage or current detected.
 20. A measuringdevice according to claim 18, further comprising a second analog todigital converter coupled to the shunt resistor detector for generatingdigital signals representative of the current flowing through saidneutral conductor and each of said at least one live conductors.
 21. Ameasuring device according to claim 20, wherein the second analog todigital converter is coupled to said resistor detector means forgenerating digital signals representative of the voltage between saidneutral and live conductors.
 22. A measuring device according to claim20, wherein the second analog to digital converter includes amultiplexer for selectively coupling two or more of said detector meansand generating corresponding digital signals representative of thevoltage or current detected.
 23. A measuring device according to claim21, including a microprocessor for determining the power consumed by theelectrical installation and operative for generating a current imbalancesignal indicative of the residual current during real time from theresidual current signal, and operative for generating a trip signal onthe basis of a comparison of the current imbalance signal with apredetermined threshold criterion.
 24. A measuring device according toclaim 16 comprising a communication device for transmitting monitoringinformation to a remote station.
 25. A combined toroid/shunt device fordetecting a residual current in an electrical installation comprising aplurality of conductors, the device comprising: a toroid means fordetecting an AC residual current lying within a first range; a pluralityof resistive shunts for connection in respective ones of said pluralityof conductors; and current detection means responsive to current flowingin each of said shunts for detecting a DC residual current and/or an ACresidual current lying within a second range.
 26. A combinedtoroid/shunt device according to claim 25 wherein the first range of ACresidual current is an AC residual current resulting from earth leakageor cross-leakage between the conductors up to a saturation level atwhich the toroid or electronic means associated therewith becomesaturated, and the second range includes AC residual currents above saidsaturation level.
 27. A combined toroid/shunt device according to claim25, wherein the conductors comprise a live conductor and a neutralconductor.
 28. A measuring device for an electrical installationcomprising a plurality of conductors, the device comprising toroid meansfor detecting a residual current and power consumption means comprisinga shunt detector means for generating a current signal indicative ofcurrent detected in at least one of said conductors and a resistordetector means for generating a voltage signal indicative of voltageacross at least one pair of said conductors.
 29. A measuring deviceaccording to claim 28, wherein the toroid means comprises a toroidaltransformer detector for detecting a residual current signal in responseto a detected residual current in said electrical installation, and saidresistor detector means is provided for connection between neutral andlive conductors of said electrical installation to generate said voltagesignal by measuring the voltage drop across the resistor means or apotentially divided portion thereof, the device further comprising aprocessor means for generating a trip signal indicative of the presenceof a residual current fault in dependence on said residual currentsignal, and an output signal derived from said current and voltagesignals to facilitate determination of power consumption.
 30. Ameasuring device according to claim 29, wherein the shunt detector meanscomprises a respective shunt connected in series in each of a pluralityof phases of an AC current supply, each phase comprising a supply via aphase live conductor and said neutral conductor, wherein the shuntdetector means is operative for providing a respective current signalindicative of the current flowing in the respective phase live conductorfor each phase of the AC supply, and said resistor detector meanscomprises a respective resistor means connected between each of saidphase live conductors and said neutral conductor, said processor beingoperative for generating signals representative of the voltage betweeneach phase live conductor and said neutral conductor by measuring thevoltage across each of said resistor means or potentially dividedportions thereof.